1. Field of the Invention
The subject invention relates to a uretonimine-modified isocyanate composition and method of forming the same. More specifically, the subject invention relates to a uretonimine-modified isocyanate composition having reduced color and a method of forming the uretonimine-modified isocyanate composition having the reduced color.
2. Description of the Related Art
Various uretonimine-modified isocyanate compositions, also known as carbodiimide-modified isocyanate compositions, are known. These compositions generally comprise a polyisocyanate composition having two or more isocyanate groups and comprising 4,4′-diphenylmethane diisocyanate (MDI) and a catalyst for catalyzing a reaction of the isocyanate groups to form carbodiimides. The carbodiimides then react with available isocyanate groups to form uretonimine and uretonimine oligomers. Once the reaction has achieved a desired level, generally based upon a percent NCO groups remaining, a quenching agent may be added to quench the catalyst used to promote the reaction of the isocyanate groups, thus inhibiting the formation of additional carbodiimides thereby inhibiting additional formation of uretonimine and uretonimine oligomers. Examples of quenching agents that have been employed include strong acids, such as trifluoromethane sulfonic acid or perchloric acid in U.S. Pat. No. 4,260,554. Other examples include weaker acids, such as Lewis acids, aromatic carboxylic acid halides, aromatic sulfonic acid esters and the like in U.S. Pat. No. 4,120,884.
Generally, it has been known to quench the reaction with a very strong acid to prevent, inhibit, or retard continued reaction of the isocyanate groups over time. However, the addition of the strong acid results in the uretonimine-modified composition becoming colored, typically yellow, orange, or brown. Any isocyanate products or articles formed from the colored uretonimine-modified composition generally have the same color. Therefore, the articles may require additional processing to disguise the color or additional amounts of pigment are necessary to overcome the prior color of the uretonimine-modified composition.
The weaker acids, while affording lower color uretonimine-modified compositions, generally do not adequately deactivate the catalyst. Therefore, the stability of the composition is undesirable and the composition must be used within a shorter period of time. If the composition is used after the stability of the composition has been compromised, then articles formed therefrom are susceptible to being rejected as non-conforming.
Attempts have been made to reduce coloration of polyisocyanate compositions. In U.S. Pat. No. 5,359,129 to Shimizu et al., a hindered amine compound is used to prevent coloration of diphenylmethane diisocyanate. However, the hindered amine compound has two phenyl groups each having a hydroxyl group, thereby resulting a basic compound. The '129 patent does not disclose utilizing the hindered amine compound with uretonimine-modified isocyanate compositions. Since uretonimine-modified isocyanate compositions include catalysts and quenching agents, adding the basic hindered amine compound would unfavorably interact with the quenching agents. As the amount of quenching agent available to deactivate the catalyst is reduced, the catalyst can further catalyze the reaction of the isocyanate groups and the stability of the uretonimine-modified isocyanate composition is reduced. Uretonimine-modified isocyanate compositions formed with the hindered amine compound of the '129 patent are likely to be unacceptable in typical manufacturing processes.
Various other stabilizers have been used with pure diphenylmethane diisocyanate, i.e., not uretonimine-modified isocyanate compositions. For example, U.S. Pat. No. 4,677,154 to Narayan et al. discloses an additive stabilizer package comprising 2,6-di-(t-butyl)-p-cresol and a compound selected from the group consisting of tris(nonylphenyl) phosphite, tetrakis(2,4-di-ti-butylphenyl)-4,4′-biphenylene diphosphonite, bis(2,4-di-t-butylphenyl)pentaerythritol disphosphite, 2,2′-oxamido-bis-distearyl thiodipropionate, triisodecyl phosphite, trilauryl trithiophosphite, distearyl pentaerythritol, 2,2′-ethylene bis(4,6-di-tertiaryburylphenol), octadecyl 3-(3′,5′-di-t-butyl4′-hydroxyphenyl) propionate, 4,4′-thio-bis(2-t-butyl-5-methylphenol), 4,4′-thio-bis(2-t-butyl-5-methylephenol), 2,2′-methylene-bis-(4-methyl-6-t-butylphenol), tris (3,5-di-tributyl4-hydroxybenzyl) isocyanurate, phenothiazine, tris(2,4-di-tbutylphenyl)phosphite, 4,4′-methylene bis (2,6-di-t-butylphenol), 2,6-di-t-butyldimethyl-aminomethylphenol and blends thereof. However, the stabilizer package is not exposed to the processing conditions of the uretonimine-modified isocyanate compositions, specifically, the catalyst and the quenching agent.
Accordingly, it would be advantageous to provide a composition that overcomes the inadequacies that characterize the related art.